What's ridiculous about using standard statistical practices when determining the effect of a change from the figures?
The standard would be average before - involving more than one year so your sample is large enough to be reliable and the same afterwards to see if there is a significant difference between the two [ i assume we have controlled for other variables]
FWIW you seem to be using a SCED and expecting instant results I assume the baddies dont always hand in the guns so it may take some time to see the benefit
It appears you don't need a gun to have a massacre.
Fantastic would you like to prove something else no one said?
Banning cars wont end "transport related deaths" either
The issue is does it reduce gun related ones and does it reduce deaths [ by murder] in general.
I think we all know banning guns does not ban massacres - it might make them a shit lot harder to do - or less "killy"* when done
* seems we are at this level of "debate"